Pressure Stepped Microwave Assisted Digestion

ABSTRACT

An instrument and method for high pressure microwave assisted chemistry are disclosed. The method includes the steps of applying microwave radiation to a sample in a sealed vessel while measuring the temperature of the sample and measuring the pressure generated inside the vessel and until the measured pressure reaches a designated set point, opening the vessel to release gases until the pressure inside the vessel reaches a lower designated set point, closing the vessel, and repeating the steps of opening the vessel at designated pressure set points and closing the vessel at designated pressure set points to the sample until the sample reaction reaches a designated high temperature. The designated set points can controllably differ from one another as the reaction proceeds. Microwave energy can be applied continuously or intermittently during the opening and closing steps. The apparatus includes a microwave cavity, a microwave transparent pressure resistant reaction vessel in the cavity, a cap on the reaction vessel, a pressure sensor for measuring pressure in the vessel, a temperature sensor, and means for opening and closing the cap at predetermined pressure set points measured by the pressure sensor to release pressure from the vessel.

BACKGROUND

The invention relates to microwave assisted chemical reactions carriedout at elevated temperatures and elevated pressures. In this context,the term “digestion” refers to the reaction of a sample with anaggressive acid (e.g., nitric, HNO₃) at high temperatures and pressures.The combination of temperature and the strong acid tends to break most,and preferably all, of the chemical bonds in the sample to produce aliquid containing the constituent species, typically elements, of thesample. The liquid can then be analyzed for the presence and amounts ofthese elements.

Microwave systems are often used to accelerate the digestion process.Microwaves typically interact directly with the digestion acid andsometimes with the sample composition and thus in many cases microwavedigestion can be carried out more quickly than digestion usingconventional heat sources. Examples include, but are not limited to U.S.Pat. No. 5,420,039, U.S. Pat. No. 4,946,797, and U.S. Pat. No.4,861,556.

Although digestion can be carried out using several different acids(e.g., sulfuric, nitric, phosphoric, hydrochloric, hydrofluoric, orperchloric), nitric acid offers advantages in some circumstances. Inparticular, nitric acid avoids forming insoluble compounds with manyinorganic samples. Other acids (e.g., sulfuric and hydrochloric) aremore likely to form such insoluble compounds during digestion reactions.Thus, nitric acid is often preferred for digestion because it produces ahigher quality sample for analytical testing.

In order to digest in HNO₃, however, many samples must typically beheated above the atmospheric boiling point of the acid; e.g., nitricacid boils at about 120° C., but many samples do not digest completelyunless heated to at least about 200° C., and some samples requiretemperatures of 250-300° C. Thus, in order to reach higher temperatures,nitric acid digestion must be carried out in a pressurized environment,typically using vessels that can withstand pressures of several hundredpounds per square inch.

In order to prevent catastrophic failure at such pressures, mostdigestion vessels include some type of release capacity. These includerupture disks or diaphragms that break at a certain pressure (e.g., U.S.Pat. No. 5,230,865). Other digestion vessels will flex to create a smallopening, for example between the body of the vessel and its lid, throughwhich the excess pressure can escape (e.g., U.S. Pat. No. 6,287,526).Other systems are described in, for example, U.S. Pat. No. 5,948,307;U.S. Pat. No. 5,204,065; U.S. Pat. No. 5,264,185; U.S. Pat. No.5,620,659 and EP0198675. These items are exemplary rather thanexhaustive or limiting.

Such pressure release systems are effective for their intended purpose,but they lack precise control over the point at which they will release.Additionally, if the vessel re-seals itself, it does so at an arbitrarypressure rather than at a controlled pressure. As another factor, allvessels are ultimately limited in their pressure capacity.

To some extent, the gas-containing capacity of a vessel can be increasedby increasing the vessel's size. Larger vessels, however, carry somecorresponding disadvantages. They require, of course, larger instrumentsto accommodate them. From a safety standpoint, the total force within avessel is a function of the pressure and the area defined by the vesselwalls. Thus, larger vessels are subject to larger total forces and carrycorrespondingly higher risks of catastrophic failure.

Furthermore, in digestion systems where pressure is not released untilthe reaction is complete (and the vessel and its contents sufficientlycooled), the vessel volume must be sufficient to contain the sample, theacid, and the gases generated by the digestion reaction at the maximumdigestion temperature.

Other pressure release systems attempt more sophisticated solutions.Legere and Salin, “Design and Operation of a Capsule-Based MicrowaveDigestion System,” Analytical Chemistry 1998, 70, pp. 5029-5036,describe an apparatus and system where a small (8.4 mm diameter, 25 mmlength) polymeric gel capsule containing a sample is inserted into aTeflon™ tube. A digestion acid is then added to the tube and the tube issealed. Microwaves are then applied to the tube, the gel capsule, theacid, and the capsule contents. The capsule breaks and the acid reactswith the sample. On a periodic basis, the application of microwaves is,however, stopped, the tube is proactively cooled with water, and excessgases are released. The technique is limited by the pressurecapabilities of the tube and by the temperature at which the capsulematerial will digest. In other words, because the capsule breaks up andmixes with the digestion acid, the digestion temperatures must bemaintained below those temperatures at which the capsule material woulddigest and add elements to the sample that would produce an improperanalysis. According to Legere, polyacrylamide provides an appropriatecapsule material, but contains trace quantities of iron, calcium,sodium, aluminum, and magnesium. Furthermore, polyacrylamide will tendto begin digesting at 230° C. As a result, the ongoing digestionreaction of the sample must be maintained sufficiently below 230° C. toavoid any digestion of the capsule and any consequent pollution of thesample results.

The Legere technique appears to have other disadvantages. As one, thereaction returns to atmospheric pressure on a repeated basis, thuseffectively cooling the sample and reducing the temperature. As anotherdisadvantage, both the described “flange valve” and the “squeegee”cleaning technique would appear to raise cross-contaminationpossibilities between and among digestion samples.

SUMMARY

In one aspect, the invention is a method of high pressure microwaveassisted chemistry. The method includes the steps of applying microwaveradiation to a sample in a sealed vessel while measuring the temperatureof the sample and measuring the pressure generated inside the vessel anduntil the measured pressure reaches a designated set point, opening thevessel to release gases until the measured pressure inside the vesselreaches a lower designated set point (which can be selected rather thanarbitrarily accepted), closing the vessel, and repeating the steps ofopening the vessel at designated pressure set points and closing thevessel at designated pressure set points until the sample reactionreaches a designated high temperature. The designated set points cancontrollably differ from one another as the reaction proceeds. Microwaveradiation can be applied (or moderated) either continuously orselectively during the overall digestion, and the reaction can bemaintained (dwell) at designated temperatures for selected periods oftime.

In another aspect, the invention is an apparatus for microwave assistedhigh pressure high temperature chemistry. In this aspect, the inventionincludes a microwave cavity, a microwave transparent pressure-resistantreaction vessel in the cavity, a cap on the reaction vessel, a pressuresensor for measuring pressure in the vessel, and means for opening andclosing the cap at predetermined pressure set points measured by thepressure sensor to release pressure from the vessel.

In another aspect, the invention is an apparatus for microwave assistedhigh pressure high temperature chemistry that includes a source ofmicrowave radiation, a microwave cavity in communication with thesource, a pressure resistant reaction vessel in the cavity, a flexiblecap on the reaction vessel, a cap seal bearing on the flexible cap, apressure sensor in pressure communication with the flexible cap, amechanical arrangement for opening and closing the cap seal and theflexible cap at predetermined pressure set points measured by thepressure sensor.

In another aspect, the invention is the combination of a pressure vesseland a venting cap. In this aspect, the invention includes a pressureresistant reaction vessel with a circular mouth at one end thereof andan annular lip extending outwardly from the mouth parallel to thecircular plane of the mouth, and a flexible cap on the mouth of thereaction vessel. The flexible cap includes a circular cover over thecircular mouth of the vessel, an annular wall surrounding the exteriorof the annular lip, an annular ring at the bottom of the annular wall,with the ring projecting underneath the annular lip toward the vesselwalls for positioning the flexible cap on the vessel and maintaining thecap in place on the vessel, at least one indentation in the circularcover for minimizing distortion when any contents of the vessel exertpressure against the lid, and at least one opening in the annular wallfor providing a ventilation path through the cap when gas pressure inthe vessel flexes the cap sufficiently to partially disengage at least aportion of the cap from the vessel.

In another aspect, the invention is a venting cap for pressure vesselsfor microwave assisted chemistry. The venting cap includes a flexiblecircular cover for closing the mouth of a reaction vessel, a flexibleannular wall depending from the circular cover, a flexible annular ringat the bottom of the annular wall and parallel to the circular cover forpositioning the cap on a reaction vessel and maintaining the cap inplace on a reaction vessel, at least one indentation in the circularcover for minimizing distortion when any contents of a reaction vesselexert pressure against the cap, and at least one opening in the annularwall for providing a ventilation path through the cap when gas pressurein a reaction vessel flexes the cap sufficiently to partially disengageat least a portion of the cap from the mouth of a reaction vessel.

The foregoing and other objects and advantages of the invention and themanner in which the same are accomplished will become clearer based onthe followed detailed description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-sectional perspective view of the interior ofan instrument according to the present invention.

FIG. 2 is a partial cross-sectional perspective view of an enlargedportion of FIG. 1.

FIG. 3 is another partial cross-sectional perspective view enlarged froma different portion of FIG. 1;

FIG. 4 is a third partial cross-sectional view enlarged from yet anotherportion of FIG. 1

FIG. 5 is a cross-sectional view of an instrument according to thepresent invention.

FIGS. 6 and 7 are additional cross-sectional views of the instrumentsimilar to FIG. 5, but illustrating slightly different positionalrelationships

FIG. 8 is a perspective view of a flexible vessel lid according to theinvention.

FIG. 9 is a cross-sectional view of the lid of FIG. 8 in its operatingenvironment.

FIG. 10 is a perspective view of a second embodiment of a flexiblevessel lid according to the invention.

FIG. 11 is a side elevation view of the lid of FIG. 10.

FIG. 12 is a cross-sectional view taken along lines 12-12 of FIG. 11.

FIG. 13 is a cross-sectional view taken along lines 13-13 of FIG. 11.

FIGS. 14, 15 and 16 are combined plots of temperature, pressure andapplied microwave power for respective digestions carried out on samplesof sugar, oil, and tea.

FIGS. 17, 18 and 19 are separate breakout versions of the plotsillustrated in FIG. 16.

DETAILED DESCRIPTION

The invention is a method of stepwise opening and closing of a digestionvessel at designated pressures to release excess gases (and thuspressure) as a digestion reaction proceeds and as the reactiontemperature increases, and without stopping the reaction and (ifdesired) without stopping the application of microwaves.

Because pressure is vented on a stepwise basis, the overall size of thedigestion vessel can be reduced. The digestion vessel can be relativelysmall because no need exists for the digestion vessel to hold all of thegases generated throughout the entire digestion reaction. Smallervessels are safer and easier to operate under pressure. The total forceexerted within the vessel is a function of the pressure multiplied bythe interior area of the vessel. Smaller vessels are thus exposed to asmaller total force. Smaller vessels also cool faster after a reactionis complete, thus reducing overall cycle time.

Periodic stepwise pressure release also prevents the contents of thevessel from generating an aerosol that would possibly carry off some ofthe elements that are intended to be measured.

FIG. 1 is a partial cross-sectional perspective view of one embodimentof an instrument according to the present invention and broadlydesignated at 20. In some of its broad aspects, the instrument includesa microwave cavity 21 which in the illustrated embodiment is generallycylindrical in shape. Those familiar with microwave instruments willrecognize that a cavity of this type is typically (although notnecessarily) cylindrical and cooperates with a microwave source (notshown) to propagate microwaves in the cavity. In exemplary embodiments,and depending upon the nature of the chemical reaction being carriedout, the combination of the microwave source and cavity 21 can produce asingle mode of microwave radiation in the cavity 21; e.g., commonlyassigned U.S. Pat. No. 6,744,024; the contents of which are incorporatedentirely herein by reference.

A pressure resistant vessel illustrated as the cylindrical vessel 22 ispositioned in the cavity and is closed with a cap (lid) 23. The reactionvessel 22 is transparent to microwave radiation and resistant tochemical attack from strong acids at elevated temperatures. In exemplaryembodiments the vessel 22 is formed of a material selected from thegroup consisting of quartz, composite materials, and polymers. In thedigestion context, quartz is often used because it is transparent in thevisible frequencies so that the completion of the reaction can beobserved or confirmed by an operator. Engineering polymers areappropriate for pressure containment and chemical resistance, but thosestrong enough to withstand high pressures are typically opaque tovisible light.

As used herein, the term “composite materials” refers to combinations ofmaterials that together provide a desired set of properties. Forexample, engineering polymers are often combined with high strengthfibers to produce a structure in which the polymer provides pressureresistance and the fiber provides a flexible matrix that minimizes oreliminates partial or catastrophic failure (typically shattering) of thepolymer. As another example, glass (which is typically inappropriate fordigestion because it can leach elements) can be coated with PTFE (oranother appropriate fluoropolymer) so that the glass portion of thevessel provides the necessary pressure strength while the fluoropolymercoating provides the desired resistance to chemical attack. Appropriatecomposite materials are well-understood in this art and will not beotherwise described in detail.

In other embodiments a liner can be used inside the vessel so that thevessel provides pressure strength and the liner provides chemicalresistance and convenience in use (e.g., when the liner is inexpensiveenough to be considered disposable).

The instrument 20 includes means for opening and closing the cap 23 atpredetermined pressure set points measured by a pressure sensorillustrated as the load cell 24.

In the illustrated embodiment, a steel block 25 and its associated partsbear against the cap 23 and is connected to an arm 26 that raises andlowers the block 25. These items will be described in more detail withrespect to FIGS. 2 and 3.

FIG. 1 illustrates a number of other elements of the instrument. Theillustrated embodiment includes a two-part housing with a first portionof the housing 27 being positioned over the cavity and the reactionvessel 22 and a second housing portion 30 being positioned rearward ofthe cavity and the vessel 22. In operation, and in a manner describedwith respect to FIGS. 5-7, the instrument includes a motor 31 which,through a drive gear 32 and a driven gear 33, turns a lead screw 34which is attached to a yoke 35. When the motor 31 moves the lead screwand yoke 35 horizontally, the yoke moves a roller 36 that rests againsta roller pad 37.

The roller 36 is in turn connected to the arm 26 which raises and lowersthe block 25 to open and close the lid 23 and the vessel 22.

FIG. 2 is an enlarged portion of the cross-sectional perspective view ofFIG. 1 illustrating the lid 23 and related items in larger detail. Itemsidentical to those in FIG. 1 carry identical reference numerals. Thus,FIG. 2 includes the cavity wall 21, the reaction vessel 22, the lid 23(the words “lid” and “cap” are used interchangeably in thisspecification), the steel block 25 and the load cell 24.

FIG. 2 also illustrates that the lid 23 is covered by a diaphragm 40which is formed of a flexible, chemical-resistant polymer, and whichhelps protect the sensitive portions of the instrument 20 from contactwith potentially corrosive vapors that may be released from the vessel22.

The steel block 25 holds a lid seal 41 in the shape of an inverted Tagainst the diaphragm 40 and the lid 23. The lid seal 41 surrounds aload transfer rod 42 which bears against the diaphragm 40 and the lid 23and which is in physical contact with the load cell 24. In thisarrangement, pressure generated by the chemical reaction in the vessel22 exerts a force against the flexible lid 23 which in turn transfersthe force through the diaphragm 40 to the load transfer rod 42 and thusto the load cell 24 which provides the desired pressure measurement insignal form. The load cell 24, also referred to as a force transducer orsensor, is a device that translates loads or forces into measurableelectrical output. Such load cells are commercially available and wellunderstood by those of ordinary skill in this part.

The cap (lid) 23 is formed of a material that will accurately transferthe pressure inside the vessel to the load transfer rod 42 and to theload cell 24, and that is resistant to chemical attack from strong acidsat elevated temperatures. The lid 23 is most often formed of a flexiblematerial, but a rigid cap will work provided that it transfers pressureto the load cell and opens to release pressure when the block 25 islifted. Silicone polymers (polydimethylsiloxane) are exemplary (but notlimiting) materials for the flexible lid 23. Other elastomeric polymersare appropriate provided that they are sufficiently flexible forpressure measurement purposes, can withstand chemical attack duringventing, and will not decompose at elevated digestion temperatures. Asset forth with respect to FIGS. 8-13, the cap 23 can also include afluoropolymer liner for direct contact with the digestion components.

A processor (not shown) is in signal communication with the pressuresensor and in communication with the arm 26 and a steel block 25 so thatthe processor can control the movement of the arm 26 and the block 25 inresponse to the pressure inside the reaction vessel.

The required programming and processor capacity is well within thecapability of a personal computer-type processor, and the use ofautomated controls and sequences is generally well understood in thisand related arts, e.g. Dorf, THE ELECTRICAL ENGINEERING HANDBOOK, 2d Ed.(CRC Press 1997).

FIG. 2 also illustrates that the lid seal 41 is positioned inside thesteel block 25 with a first spring 43 bearing against both the block 25and the lid seal 41 and a second spring 44 bearing against the lid seal41 and a second lid seal member 45. The springs 43 and 44 exert a fixeddownward pressure against the lid 23. Because the lid 23 is maintainedunder the force of the springs, the lid 23 moves against the loadtransfer rod 42 only in response to pressure increases within the vessel22. As a result, the signals from the pressure sensor (load cell) 24will accurately reflect the pressure in the vessel 22 rather thanextraneous movement of the lid or other elements that may be unrelated(or not directly proportional) to the pressure in the vessel 22.

The use of both the first lid seal 41 and the second lid seal 45provides flexibility in the size of the vessels that the instrument canhandle. In the illustrated embodiment, the larger T-shaped lid seal 41and its associated spring 43 can cover the flexible lid 23 of a largervessel; typically on the order of 35 milliliters (mL). For smallersamples, the second lid seal member 45 and its associated spring 44 cancover and bear against a smaller lid on a smaller diameter vessel;typically on the order of 10 mL. The use of two separate lid sealmembers is optional rather than mandatory. The number of different-sizediameter vessels that could be incorporated under different lid sealsis, of course, conceptually unlimited. In most cases, however, thedesign will be based on practical considerations and will balance thecomplexity of the structure against the advantages of its flexibility inoperation.

The bottom portion of the load transfer rod 42 is surrounded by a steelring 48 (FIG. 9) with a square cross-section. The steel ring 48 helpsestablish a definite area across which the load transfer rod 42 and theload cell 24 measure the pressure. Defining the fixed pressuremeasurement area using the ring 48 increases the overall accuracy of thepressure measurement step.

The respective springs 43 and 44 also provide a backup againstcatastrophic failure. Thus, although the instrument and its operationare designed to continually control the pressure using the steppedrelease of gas from the vessel, if circumstances should arise in whichthe pressure increases out of control, the springs permit the lid seal41, or the second lid seal 45, or both to move in response and vent thevessel very rapidly. In such a circumstance, even though control over asingle reaction sample might be lost, the pressure release will preservethe instrument for future operation.

FIG. 2 also illustrates that the block 25 includes an ear 46. A clevis47 is attached to the ear 46 and forms part of the arm 26. Thisarrangement lifts the block 25 when the arm 26 moves in response tomovement of the yoke 35 (FIG. 1). As a result, movement of the clevis 47on the ear 46 raises and lowers the block 25 and opens and closes theflexible lid 23 on the vessel 22.

FIG. 2 further illustrates that a microwave attenuator 50 forms at leastpart of the microwave cavity 21. The reaction vessel 22 is supported inthe opening defined by the attenuator 50. As FIG. 2 illustrates, theattenuator 50 rests in an annular channel 53 formed in an upper wall 51adjacent the microwave cavity. This permits the attenuator 50 to beeasily removed and replaced which in turn makes it relatively easy touse differently-sized vessels in the cavity and in the attenuatoropening. Such a removable attenuator is described in commonly assignedU.S. Pat. No. 6,607,920; the contents of which are incorporated entirelyherein by reference.

A retaining bolt 54 helps maintain the load cell 24 in contact with theload transfer rod 42 and the lid seal 41. The interior threads on theretaining bolt 54 correspond to similar threads on the exterior of theupper portion of the lid seal 41.

A vent seal housing 55 is positioned on the attenuator 50 and adjacentthe steel block 25. The vent seal housing 55 includes a port 56 thatprovides gas communication with a channel broadly designated at 57 inthe attenuator 50. When pressure is released from the vessel 22(including through a portion of the lid 23 that is best described withrespect to FIGS. 8-13), the escaping gases enter the channel 57 and canbe removed through the port 56. For purposes of clarity, the figuresillustrate only a portion of the vent seal 55. In actual practice, thevent seal 55 completely surrounds the vessel 22 and the attenuator 50and includes a second port. Thus, where desired or necessary, a purgingor carrier gas can be added to (or removed through) the port 56 (or thesecond port) and the channel 57. In order to both resist chemical attackand provide shielding to the microwave mode in the cavity, the vent seal55 is typically formed of a conductive polymer such as polyethylenecarrying graphite particles. A pair of O-rings 60 helps provide a sealbetween the vent seal 55 and the wall of the channel 57.

If desired, the gases that reach the channel 57 can be collected andanalyzed. In typical digestion schemes, however, the gases arepredictable (CO₂, H₂O and various N_(x)O_(y) species from the nitricacid) and thus offer little or no information about the sample.

FIG. 3 is another enlarged perspective partial cross-sectional viewtaken from a portion of FIG. 1 and generally oriented above the view ofFIG. 2. FIG. 3 illustrates that the roller 36 moves against the rollerpad 37 when the yoke 35 reciprocates horizontally in a yoke holder 61 asthe yoke 35 is driven by the lead screw 34. In the orientation of FIGS.1-3, movement of the roller 36 towards the left represents an openingmovement and movement of the roller 36 towards the right indicates aclosing movement. This will be described in more detail with respect toFIGS. 5-7.

FIG. 4 is another enlarged partial cross sectional perspective viewtaken from FIG. 1 and highlights the motor 31 and its associatedelements that opens and closes the vessel. The motor 31 is typically anelectric motor that turns a shaft 62 which is attached to a drive gear32. The drive gear 32 propels a driven gear 33 which is fastened to thelead screw 34 with an appropriate nut 63 or equivalent fixture. Thedriven gear 33 has an annular shaft 64 which turns within a ball orroller bearing 65 which is held in place by a washer 66. The lead screw34 is moveably attached to the yoke 35 with a second nut 67 so thatrotaing the lead screw 34 moves the nut 67 and the yoke 35.

FIGS. 5, 6 and 7 are similar (although not identical) cross sectionalviews of the instrument 20. The elements illustrated in FIGS. 5, 6 and 7are identical, but the three figures show the mechanical arrangementthat opens and closes the lid 23 in three different positions.

FIG. 5 illustrates the instrument 20 with the steel block 25 and the lidseal 41 in a completely open position. In the open position, the lidseal 41 is raised well above the flexible lid 23 on the vessel 22. FIG.5 illustrates the vessel 22 in complete cross-section and with a sample70 schematically indicated at the bottom of the reaction vessel 22.

In the open position of FIG. 5, the roller 36 is in its left-mostposition with respect to the housing 27, a position in which itsrelationship with the ear 46 and clevis 47 (not shown in FIG. 5) liftthe block 25 and the lid seal 41 into the indicated position.

FIG. 5 also illustrates (in cross-section) a pinion motor 71 immediatelybeneath the motor 31. The pinion motor 71 drives a rack and pinion thatmoves the housing 27 and the mechanical opening and closing arrangementlaterally as a whole from its position indicated in FIG. 5 to aretracted position (not shown) underneath the second housing portion 30.Portions of the pinion 72 are visible in FIG. 5 and several of the otherdrawings. When the first housing portion 27 and the associated partsretract into the second housing portion 30, the attenuator 50 isexposed, and the vessel 22 can be easily removed and replaced. Theattenuator 50 can likewise be replaced to accommodate a different sizevessel 22 or a vessel with a different sized neck.

FIG. 5 also illustrates a vent opening to the cavity, shown as thecurved tube 49. A fluid (e.g., air or an inert gas) can be directedthrough the tube 49 and into the cavity 21 to cool the vessel 22 and thesample 70 in a manner described herein with respect to the method.

FIG. 5 also illustrates an access port 28 into which an appropriatethermal probe (not shown) can be positioned to measure the temperatureof the vessel 22 and the sample 70. A non-contact measuring device suchas an infrared temperature detector is appropriate for this purpose.Other temperature measuring devices can be used provided they obtain anaccurate temperature measurement and do not otherwise interfere with thepropagation of microwaves within the cavity 21. The processor is insignal communication with the microwave source and can use the measuredtemperature or the measured pressure to start, stop or change thepropagation of microwave energy into the cavity and thus to the sample.

The remaining elements in FIG. 5 are otherwise the same as thoseillustrated in FIGS. 1-4.

FIGS. 6 and 7 are otherwise identical to FIG. 5, but show the roller 36progressively moving to respective partially closed (FIG. 6) and fullyclosed (FIG. 7) positions. As FIG. 6 indicates, as the roller 36 movesto the right, the steel block 25 and the lid seal 41 move downwardlytowards the vessel 22 and the flexible lid 23. As illustrated in FIG. 7,when the roller 36 moves to a position directly above the vessel 22, thearm 26 is completely vertical and the steel block 25 and the lid seal 41rest directly against the flexible lid 23 thereby sealing the lid 23against the vessel 22.

It will be understood that although FIGS. 5-7 illustrate the completeopening and closing movement, the invention includes the specificadvantage that the instrument can also carry out smaller, incrementalopening and closing movements; i.e., to stepwise lower the pressure inthe vessel 22 while nevertheless maintaining the pressure aboveatmospheric pressure.

FIGS. 8-13 illustrate aspects of the flexible version of the lid 23. Inparticular, FIG. 9 is a cross-sectional view showing the lid 23 incontext on the vessel 22.

FIG. 8 is a perspective view of the flexible lid 23. The lid 23 includesat least one indentation illustrated in FIG. 8 as the outer annularindentation 73. In this embodiment, the lid 23 also includes acorresponding inner annular indentation 74. As illustrated in FIG. 9,lower portions of the lid seal 41 include one or more depending flanges75. In particular, FIG. 9 illustrates that the second lid seal member 45also includes a depending flange 76. In the illustrated embodiment, thedepending flanges 75 and 76 are annular and match the (one or more)annular indentations on the lid 23.

The annular indentations 73, 74 and the corresponding flanges 75, 76help maintain the flexible lid 23, the lid seal 41, the other elementsof the mechanical pressure release arrangement in a desired pressuresealing and release relationship with the vessel 22. In particular, whenthe lid is formed of a material such as a silicone polymer(polydimethylsiloxane) the characteristics of the polymer can encourageit to spread laterally or distort in undesired or unintended directions.Thus, in the orientation of the lid 23 illustrated in FIG. 9, pressureinside the vessel 22 would normally tend to cause the lid 23 to distortboth vertically and horizontally (i.e, radially) under pressure. Asnoted earlier, however, in order to obtain an accurate measurement ofthe pressure inside the vessel 22, the lid 23 must bear properly againstthe load transfer rod 42. The relationship between the depending flanges75, 76 and the annular indentations 73, 74 helps maintain the lid 23 ina proper pressure exerting relationship against the load transfer rod 42because the flanges minimize or eliminate lateral movement of the lidportions under high pressure in the vessel 22.

The flanges 75, 76 also help increase the efficiency of the seal betweenthe lid 23 and the vessel 22 by exerting the applied downforce over asmaller area (i.e., where a flange meets an indentation).

The indentations are most valuable in the context in which a needle (notshown) is used to pierce the cap; for example to sample a gas or aliquid during the reaction. In other circumstances, and particularly ifthe lid 23 is not pierced, the indentations can be omitted.

FIGS. 10-13 illustrate another embodiment of the lid broadly designatedat 80. The illustrated lid 80 is substantially identical to theillustrated lid 23 with the only difference being that the lid 80includes only one annular indentation 81. Some of the other features ofthe lid are thus more clearly illustrated with respect to the lid 80.For example, both FIGS. 8 and 10 illustrate a vent opening 82 in therespective lids 23 and 80.

As illustrated in FIG. 10, the lid 80 includes a circular cover 83 whichin use (e.g., FIG. 9) is positioned over the circular mouth of thevessel 22. An annular wall 84 surrounds the exterior of an annular lip85 (FIG. 9) which is a typical and useful feature of exemplary reactionvessels such as the illustrated vessel 22. The lid 80 further includesan annular ring 86 (FIG. 12) at the bottom of the annular wall 84 withthe ring 86 projecting underneath the annular lip 85 of the vessel 22and toward the wall of the vessel 22 for positioning the lid 23, 80 onthe vessel 22 and maintaining the lid 23, 80 in place on the vessel.

The well 84 and the ring 86 help maintain the cap 23 in place on thevessel 22, form a seal with the attenuator 50, and cushions the lip ofthe vessel 22 where it meets the attenuator 50.

FIG. 11 is a side elevation view of the lid 80 illustrating the annularwall 84 and the vent opening 82. FIG. 12 is a cross sectional view takenalong lines 12-12 of FIG. 11 and illustrates that the vent opening 82extends entirely through the annular wall 84 of the lid 23, 80. FIG. 13is a cross-sectional view taken along the lines 13-13 of FIG. 11 andlikewise illustrates the position of the vent opening 82. FIG. 13 alsoillustrates that if desired, a portion 87 of a more chemically robustmaterial such as a fluorocarbon polymer (e.g., PTFE) can form theinterior of the lid 23, 80. Alternatively, the fluorocarbon can beincluded as a separate piece; i.e., a circular disc. As recognized bythose familiar with these materials, the silicone polymers arechemically quite robust, but the fluorocarbon polymers are in many casesthe most robust available for resisting attack from harsh chemicals suchas the acids used in the high temperature digestion context of thepresent invention.

Accordingly, under pressure, when the mechanical arrangement is used toslightly lift the lid seal 41, gas pressure can and will distort the lid23, 80 in a correspondingly slight manner to allow gases to escapethrough the opening 82 into the venting channel 57 described withrespect to FIG. 2 and illustrated in cross-section in FIG. 9.

The flexible lid 23 provides advantages over certain valve-type systems(e.g., Legere, supra). Using the invention, an individual lid can bemaintained with a single vessel. Thus, when a new sample is digested,the reaction can be carried out in a fresh (clean) vessel with a freshcap, thus entirely avoiding the cross-contamination potential thatexists when a common valve or vessel are exposed to many differentsamples.

In another aspect, the invention is a method of high pressure microwaveassisted chemistry. In this aspect, the invention includes the steps ofapplying microwave radiation to a sample in a sealed vessel whilemeasuring the temperature of the sample and measuring the pressuregenerated inside the vessel and until the measured pressure reaches adesignated set point above atmospheric pressure. The vessel is thenopened to release gases until the measured pressure inside the vesselreaches a lower designated set point above atmospheric pressure; i.e., apressure lower than the initial designated set point. The vessel is thenclosed at the lower designated pressure set point. These steps—openingthe vessel at designated pressure set points and then closing the vesselat other designated set points—are repeated until the sample reaches adesignated high temperature.

The microwave radiation can be applied continuously during the venting(open and closing) steps or in time-limited or power-limited intervalsas may be desired or necessary.

As set forth with respect to the instrument, the invention isparticularly useful for carrying out digestion reactions usingconcentrated nitric acid. The microwave radiation is initially applieduntil the pressure reaches a designated set point above atmosphericpressure.

As set forth with respect to the instrument, in some circumstances it ishelpful or necessary to apply a single mode of microwave radiation tothe sample and as those familiar with the propagation of microwaves arewell aware, a single mode can be propagated by properly matching asource (i.e., frequency or impedance or both) to the cavity shape andsize and tuning (if necessary) the cavity appropriately. Microwavesources suitable for digestion reactions are widely available andwell-understood in this art. Magnetrons, klystrons and solid statedevices are all appropriate in the digestion context.

Where advantageous, the steps of opening and closing the vessel can becarried out at temperatures above the atmospheric boiling point of theacid. For digestion reactions, temperatures sufficient to break down thechemical bonds of the sample will be required. If this temperature ishigher than the atmospheric boiling point of the acid, the sample willfail to digest unless the pressure can be increased sufficiently toraise the boiling point of the acid.

In the method, the vessel can be closed while the pressure continues toremain above atmospheric pressure; i.e., the method does not require anequilibrium between the ambient pressure and the pressure inside thevessel nor does it require the contents of the vessel to reachatmospheric pressure during the venting steps. Instead, the method canselectively reduce the pressure sufficiently to prevent pressure-relatedfailure of the vessel or the instrument while nevertheless continuing tocarry out the reaction at or above atmospheric pressure.

Furthermore, the set points need not be identical as the reactionproceeds; a feature that is unavailable using static devices such assprings, frames or diaphragms. Instead (and as set forth in the Examplesherein), different set points can be programmed for different pressures,time intervals, or temperatures as the reaction proceeds.

The method can also include the step of measuring the temperature of thevessel and the sample during any of the microwave-applying,vessel-opening and vessel-closing steps. In turn, the method can includemoderating the application of microwave energy (typically on atime-limited or power-limited basis) in response to the measuredtemperature. As used herein, the phrase, “moderating the application ofmicrowave energy,” can include initiating, changing, or stopping theapplication of microwaves.

The method can further comprise thermally managing the reactiontemperature, with an exemplary method including the step of proactivelycooling the reaction vessel, usually by directing a fluid into contactwith the vessel, during any one or more of the steps of applyingmicrowave radiation, opening the vessel, or closing the vessel.

In the method of the invention, the microwave radiation is applied (andif desired, moderated) based on the measured temperature. The ventingsteps are carried out based upon the measured pressure and thus can beindependent of the application of microwave energy and independent ofthe measured temperature.

EXAMPLES

FIGS. 14-19 illustrate the progression of pressure, temperature andmicrowave power during exemplary digestion reactions. FIG. 14 plots thedigestion of a sugar sample, FIG. 15 an oil sample, and FIG. 16 a tealeaf sample. These are exemplary of different types of materialsanalyzed using digestion. Sugar, being an organic molecule, will tend togenerate a relatively large volume of gas. Oil is a hydrocarbon liquidthat requires robust conditions before breaking down. Tea leavesrepresent vegetable organic material.

For each of these examples, the indicated amount of sample was combinedwith 10 mL of concentrated (68%) nitric acid. The instrument was set(programmed) to generate a five minute ramp from the startingtemperature (e.g., 25° C.) to 200° C. followed by a three minute hold at200° C. Based on the feedback controls in the instrument, the instrumentautomatically adjusts the power to follow this temperature protocol.

Independently of the temperature, the instrument was programmed to ventin the following manner:

The sugar sample was programmed for two vent openings at 100 pounds persquare inch (psi), two at 160 psi, two at 220 psi, two at 250 psi andthen as many as necessary at 280 psi.

For the tea (FIG. 16) and the oil (FIG. 15), the venting was programmedfor two openings at 200 psi, two at 260 psi, two at 280 psi, and as manyas necessary at 300 psi.

The reaction, of course, does not necessarily reach all of theprogrammed pressure points. Thus, FIG. 15 illustrates that the oilsample reached 300 psi and was vented (to 250 psi) whenever it reached300 psi. The sugar sample (FIG. 14) reached a maximum pressure of about225 psi, and the tea sample (FIG. 16) reached a maximum of about 275psi.

In each of the vent steps for each of the samples, the vessel was openedat a pressure above atmospheric pressure and then closed at a lowerpressure, but one that remained above atmospheric pressure. Thus, whenthe sugar was vented at 100 psi, the vessel was closed when the pressuredropped to 50 psi. Likewise, when the pressure reached 160 psi, ventingwas carried out until the pressure drop to 100 psi and when the pressureexceeded 220 psi, venting was carried out until the pressure dropped toapproximately 175 psi. Similar protocols were followed for the samplesillustrated in FIGS. 15 and 16 and the pressure plots demonstrate this.

As each of FIGS. 14, 15 and 16 indicate, as microwave power is applied,the temperature and pressure both increase. The pressure increases,however, are accompanied by the specific release steps of the method ofthe invention. When illustrated in FIGS. 14-16, the pressure releasesteps appear as the jagged up-and-down entries in the respective dashedlines. The temperature (solid line) tends to increase more smoothlybased on the manner in which the instrument is programmed and then canbe maintained at a desired set point (200° C. for these samples) untilthe reaction is complete. When the reaction is completed, the sample isallowed to cool. As set forth earlier, the microwave power (dotted line)is adjusted based on the difference between the measured temperature andthe desired (programmed) temperature. The venting can take placeindependently of the temperature based on the measured pressure.

In the drawings and specification there has been set forth a preferredembodiment of the invention, and although specific terms have beenemployed, they are used in a generic and descriptive sense only and notfor purposes of limitation, the scope of the invention being defined inthe claims.

1. The combination of a pressure vessel and a venting cap, thecombination comprising: a pressure-resistant reaction vessel with acircular mouth at one end thereof and an annular lip extending outwardlyfrom said mouth parallel to the circular plane of said mouth; a flexiblecap on said mouth of said reaction vessel; said flexible cap comprising,a circular cover over said circular mouth of said vessel; an annularwall surrounding the exterior of said annular lip; an annular ring atthe bottom of said annular wall, said ring projecting underneath saidannular lip toward said vessel walls; at least one indentation in saidcircular cover for minimizing distortion when any contents of saidvessel exert pressure against said cap; and at least one openingextending through said annular wall for providing a ventilation paththrough said cap when gas pressure in said vessel flexes said capsufficiently to partially disengage at least a portion of said cap fromsaid vessel.
 2. The reaction vessel of claim 1 wherein said vessel isformed of material transparent to microwave radiation.
 3. The reactionvessel of claim 1 wherein said vessel is formed of material resistant tochemical attack from strong acids at elevated temperatures.
 4. Thecombination according to claim 1 wherein said annular ring at the bottomof said annular wall comprises a flexible annular ring.
 5. Thecombination according to claim 1 wherein said annular ring at the bottomof said annular wall comprises a ring parallel to said circular cover inan anchoring relationship to said cap on said reaction vessel.
 6. Thecombination according to claim 1 wherein said indentation in said covercomprises an annular indentation.
 7. The combination according to claim1 comprising a plurality of indentations in said cover.
 8. Thecombination according to claim 7 comprising a plurality of annularindentations in said cover.
 9. The combination according to claim 1comprising a cylindrical reaction vessel.
 10. The combination accordingto claim 1 wherein said reaction vessel is formed of a material selectedfrom the group consisting of quartz, composite materials and polymers.11. A combination according to claim 1 comprising a vessel formed offluoropolymer coated glass.
 12. A combination according to claim 1comprising a chemical resistant liner inside said vessel.
 13. Thecombination according to claim 1 wherein said cap is formed of a poly(dimethylsiloxane) polymer.
 14. A venting cap for pressure vessels formicrowave-assisted chemistry comprising: a flexible circular cover forclosing the mouth of a reaction vessel; a flexible annular walldepending from said circular cover; a flexible annular ring at thebottom of said annular wall and parallel to said circular cover forpositioning said cap on a reaction vessel; at least one indentation insaid circular cover for minimizing distortion when any contents of areaction vessel exert pressure against said cap; and at least oneopening in said annular wall for providing a ventilation path throughsaid cap when gas pressure in a reaction vessel flexes said capsufficiently to partially disengage at least a portion of said cap fromthe mouth of the reaction vessel.
 15. A venting cap according to claim14 comprising an elastomer.
 16. A venting cap according to claim 14wherein said cap comprises a material that accurately transfers thepressure inside the vessel to a load transfer rod and to a load cell.17. A venting cap according to claim 14 wherein said cap comprises amaterial resistant to chemical attack from strong acids at elevatedtemperatures.
 18. A venting cap according to claim 15 wherein saidelastomer comprises polydimethylsiloxane.
 19. A venting cap according toclaim 14 further comprising a fluoropolymer liner beneath said circularcover.
 20. A venting cap for pressure vessels for microwave-assistedchemistry comprising: a rigid circular cover for closing the mouth of areaction vessel; a flexible annular wall depending from said circularcover; a flexible annular ring at the bottom of said annular wall andparallel to said circular cover for positioning said cap on a reactionvessel; at least one indentation in said circular cover for minimizingdistortion when any contents of a reaction vessel exert pressure againstsaid cap; and at least one opening in said annular wall for providing aventilation path through said cap when gas pressure in a reaction vesselflexes said cap sufficiently to partially disengage at least a portionof said cap from the mouth of the reaction vessel.